A clutch device has been known conventionally to transmit a rotational driving force of a power source to a drive wheel by a frictional force of a friction plate (clutch plate) which is drive-controlled by an actuator. In such a clutch device, for example, when the clutch plate is worn out and becomes thin, the same frictional force may not be acquired even if a drive amount of the actuator is the same as before. Such a problem can be solved by, for example, detecting a movement amount required for clutch plates, which are apart from each other, to come into contact with each other, and by increasing the drive amount of the actuator in accordance with an increase of this movement amount.
Japanese Patent Application Publication No. 2004-197842 (Japan '842) generally discusses a structure in which a movement amount required for clutch plates to come into contact with each other is detected on the basis of a position of a member displaced by an actuator.
However, in the structure discussed in Japan '842, since a wearing degree of the clutch plates is estimated on the basis of the drive amount of the actuator, it has been impossible to detect a change of the frictional force caused by changed friction characteristics of the clutch plates or changed characteristics of an oil which lubricates the clutch. Meanwhile, in a structure in which a pressing force for connecting a clutch is obtained not by an elastic member, such as a clutch spring, but by a frictional force of an actuator and an oil pressure controlled by the actuator, the frictional force is changed especially by the pressing force after clutch plates come into contact with each other. Therefore, in order to perform more suitable clutch control, there has been a demand on detecting timing at which the clutch actually starts to be engaged.